Crabwise · Library
Which fuel quantity belongs in an endurance calculation?
Forty units at 8 per hour divide to 300 minutes, but removing 3 unusable units and a 45-minute reserve of 6 leaves 31 units and 232.5 minutes.
The division uses exactly what you type
Crabwise's endurance mode computes t = F ÷ FF × 60. It has no tank model and no separate field for unusable fuel or reserve, so the fuel you enter is the fuel it divides. The Library worksheet labels the same field fuel available and notes that the result does not include a reserve unless you subtract one.
Decide what the number means before typing it: total tank contents, usable fuel from the aircraft's documents, or usable fuel minus a reserve you are holding back.
Worked example
The division gives 180 minutes for 24 units at 8 units per hour. It has not established what portion of a real tank’s contents those 24 units represent.
- Time from the entered quantity and flow (min)
- 180
Prepare the fuel figure
- Start from the quantity on board in one unit.
- Subtract the unusable quantity stated for the aircraft.
- Convert your reserve minutes to fuel at the planned flow and subtract it if the endurance should exclude it.
- Enter the result with the same flow and note which deductions were made.
Gross and net endurance
The worksheet's default of 40 units at 8 units per hour gives 40 ÷ 8 × 60 = 300 minutes. If 3 units are unusable and a 45-minute reserve is held back, that reserve is 8 × 45 ÷ 60 = 6 units. The figure to divide is 40 − 3 − 6 = 31 units, which lasts 232.5 minutes.
- Endurance from 40 units (min)
- 300
- Reserve for 45 min at 8 per hour
- 6
- Fuel after deductions
- 31
- Endurance after deductions (min)
- 232.5
Taking out only the 3 unusable units leaves 37 units and 37 ÷ 8 × 60 = 277.5 minutes. The step from 300 to 277.5 to 232.5 minutes shows how much each deduction moves the result.
Common mistakes
- Entering full tank capacity and reading the result as usable endurance.
- Subtracting the reserve twice, once from the fuel and again from the minutes.
- Mixing a quantity in litres with a flow in US gallons per hour.
- Using a flow measured in cruise for an endurance that includes climb.
Scope
The minutes are arithmetic, not a measured tank reading or an operating limit. The FAA navigation chapter below covers fuel and time.
Educational calculation only. Check current aircraft limits, weather, performance and reserve requirements with approved information.